A transmission is a device that converts a power of an engine into an adequate torque according to the speed and load of a vehicle. The transmission is classified into a manual transmission and an automatic transmission according to its speed change type, and most of the sedans currently produced are mounted with the automatic transmission.
The automatic transmission is a transmission whose speed change operation is automatically performed in a predetermined pattern. Such an automatic transmission includes a torque converter; an actuator; a planetary gear system; a hydraulic control unit; and an electronic control unit. The hydraulic control unit is provided with a hydraulic pressure control valve unit to control a hydraulic pressure to be supplied to a clutch and the like.
A solenoid valve serves as one of pressure control valve units mounted in the hydraulic control unit of an automatic transmission
The solenoid valve is classified into a spool type, a ball type, a poppet type and the like according to its internal structure. A spool type solenoid valve obtains a control pressure by appropriately adjusting the pressure of oil supplied from an oil pump using a spool.
However, the control pressure adjusted by the spool is largely set in consideration of a maximum load pressure that may be required in the speed change operation. Accordingly, this control pressure remains as an excessive pressure in a typical speed change operation requiring no maximum load pressure. Such a control pressure remaining as the excessive pressure makes the vehicle power transferring efficiency deteriorated.
In the meantime, a ball type or poppet type solenoid valve can perform a fine control of the control pressure, thereby obtaining a higher power transferring efficiency than that of the spool type solenoid valve. However, in the ball type or poppet type solenoid valve, there is some drawbacks, i.e., the complicated structure and the manufacturing process that is not easy to perform, since it is necessary to additionally provide an orifice in an inflow port to which oil is supplied.